Vent gas control system

ABSTRACT

A process and apparatus for reacting a gas and a liquid by selectively adjusting the vent gas rate in relation to the liquid feed rate, impurities in the gas feed and impurities in the vent gas.

Rothstein VENT GAS CONTROL SYSTEM Aug. 21, 1973 OTHER PUBLICATIONS [75]inventor: Mark Rothstem Pennsvlne Control of Reactors lnvolving GasRecycle and Purge [73] ASSignee: E- I. dll POM d Nemours and Streams byCarter, Control, November 1964, pp.

Company, Wilmington, Del. 561-565.

[22] Filed: Jan. 6, 1971 Primary Examiner-Eugene G. Botz [21] Appl104218 Attorney-Raymond E. Blomstedt [52] US. Cl. 235/15l.l2, 23/253 A,260/580 51 Int. Cl. C07c 85/10 [57] ABSTRACT [58] Field of Search 235/151.12; A process and apparatus for reacting a gas and a liquid 23/253 A;260/580 by selectively adjusting the vent gas rate in relation to theliquid feed rate, impurities in the gas feed and im- [56] References andpurities in the vent gas.

UNITED STATES PATENTS 3,032,586 5/1962 Dierichs et al 260/580 6 Chums 1Drawmg F'gure /COMPUTER l A T to SEPARATORJ l l l l l 1 GAS 1 ANALYZER 2mk,

Y i I (SEPARATOR HEAT EXCHANGER Patented Aug. 21, 1973 *SEPARATOR CO P TR M U E GAS ANALYZER SEPARATOR HEAT EXCHANGER GAS ANALYZER REACTORINVENTOR MARK B. ROTHSTEIN VENT GAS CONTROL SYSTEM BACKGROUND OF THEINVENTION This invention relates to a process and apparatus for reducingthe amount of reactant gas vented in order to purge impurities during aprocess for reacting a gas with a liquid.

In industrial processes in which a liquid is reacted with a gas, it isdesirable to maintain the overall rate of reaction with some minimumacceptable value. In accordance with the well-known mass-actionprinciples, achievement of such a reaction rate requires a high gaspurity in the reactor. Maintaining the reactant gas purity in thereactor above an acceptable minimum is usually accomplished byperiodically or continuously venting impure reactant gas from thereactor and replacing it with purer reactant gas. In reacting hydrogenwith dinitrotoluene, for example, it is desirable to control thehydrogen concentration in the vent gas at 70-90 mole percent when usinga feed-hydrogen purity of 99.5 percent. If the hydrogen concentration ofthevent gas exceeds this amount, an undue loss of hydrogen in the ventgas results, adding to the cost of the product.- At lower hydrogenconcentration on the other hand the reaction proceeds too slowly andinefficiently with resultant higher costs.

Known methods for controlling the rate of reactant gas vented includeboth manual and automatic systems. Manual procedures are ineffective,uneconomical and slow. Automatic feedback systems, those involvinganalysis of vented gas and proportionate adjustment of the gas feed rateand gas vent rate are unsatisfactory due to the process time constantfrom an upset (e.g., in the purity of the gas feed) and the firstindication of the upset in the form of a change in the vent gascomposition. With such systems impurity concentration in the reactorfluctuates over an unacceptably wide range. There has been a need for aprocess which would anticipate feed supply upsets and control thereaction conditions so that the purity of reactant gas in the reactor ismaintained within a narrow range corresponding to efficient utilizationof reactants and production of product at an efficient rate.

SUMMARY OF THE INVENTION This invention provides a process and apparatusfor reacting a liquid and a gas under controlled conditions. Thisinvention consists essentially of a process and apparatus forcontrolling the amount of reactant gas (e.g., hydrogen) vented duringthe reaction of the gas with a liquid (e.g., dinitrotoluene) in a closedreactor having a gas feed, a liquid feed, separate liquid and gas phasesexisting in the reactor, a liquid take-off and a gas takeoff by:

a. measuring the liquid feed rate to the reactor and transmitting asignal representative of this rate,

b. analyzing the gas feed for impurities and transmitting a signalrepresentative of the concentration of impurities in the gas feed,

c. analyzing the impurities in the gas phase of the reactor andtransmitting a signal representative of the concentration of impuritiesin the gas phase of the reactor,

d. receiving said transmitted signals in an analog computer programmedto determine the gas venting rate (V) required to adjust the gas purityin the reactor to a predetermined level according to the equation:

where: v

F dinitrotoluene feed rate (e.g., 4,540 Kg./hr.)

X l actual impurity concentration in the feed hydrogen (0.5 mole percentnormally) X actual impurity concentration of the vent gas (13.0 molepercent at normal steady state) e error difference between the desired(normal) and the actual impurity concentration of the vent gas (e=0 forsteady state conditions) in mole percent K controller gain (percent pervolt) y process time constant of gas contained in the reactor and thevent gas system (minutes) t time (minutes) V vent gas flow rate onwater-wet basis (33.3

KgJhr. at normal steady state) and transmitting a signal from thecomputer representative of the resulting vent gas flow rate, to a gasvent controller to adjust the set point of the vent gas flow rate to therate determined by the computer.

This invention also provides the control system necessary to operate theprocess.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a schematic sketch of aprocess for hydrogenating dinitrotoluene utilizing the vent controlsystem of this invention.

DETAILS OF THE INVENTION This invention will be described as it appliesto a preferred embodiment of the invention, the hydrogenation ofdinitrotoluene, but it is also useful for other liquidgas reactions.

Referring to the drawing, hydrogen (H gas and liquid dinitrotoluene(DNT) are fed into reactor 1 through conduits 3 and 4, respectively.Vent gas leaves the reactor through conduit 5, enters separator 6 wheresmall amounts of tolylene diamine and water are separated from the gas,and is vented through conduit 7 through orifice plate 22 which measuresthe vent gas flow rate. A flow rate signal from 22 is sent viatransmitter 23 to flow controller 24 which sends a signal representativeof such rate to both an analog computer 17 and vent gas flow controlvalve 25. Liquid tolylene diamine is drawn from the bottom of thereactor through conduit 8, pump 9 and conduit 10 and recirculatedthrough conduit 11, heat exchanger 12 and conduit 13 into the top of thereactor, but a minor portion of the liquid stream is drawn off throughconduit 14 as product.

The control system of this invention involves measuring 1) thedinitrotoluene feed rate, (2) the concentration of impurities in thehydrogen feed, and (3) the concentration of impurities in the gas phaseexisting in the reactor and transmitting signals representative of thesemeasurements to an analog computer which uses this information to solveprogrammed equation (I). The computer issues a signal representing therate at which gas must be vented from the reactor in order to offsetvariations from normal steady state operations which occur or tomaintain such operations in the absence of upsets, as the case may be. a

The dinitrotoluene feed rate to the reactor is sensed by orifice plate15 and a signal representative of this flow rate is transmitted by flowtransmitter 16 to analog computer 17. The hydrogen feed to the reactoris analyzed by a Thermatron gauge 18 manufactured by the Mine SafetyAppliance Company and a signal representing the concentration ofimpurities is sent to computer 17. The hydrogen feed rate is also usedto control the pressure in the reactor 1 by the pressure transmitter 19and the pressure controller 20. in controller 20, the reactor pressureis compared with a manually adjusted set point which controls thehydrogen feed flow rate valve 26 so that the indicated difference isreduced to zero by controller 20. The gas phase in the reactor isanalyzed for impurities by a second Thermatron analyzcr 2] locateddownstream of separator 2 in a line provided for this measurement.Thermatron analyzer 21 sends a signal representing the concentration ofimpurities in the gas phase to computer 17.

Computer 17 is programmed to use the information supplied to solveequation (1) above.

Under normal steady state operating conditions for hydrogenatingdinitrotoluene utilizing hydrogen gas containing 0.5 mole percent ofimpurities (mostly methane but possibly other gases inert toward the hydrogenation reaction), a hydrogen gas feed rate of 321 Kg./hr. and adinitrotoluene feed rate of 4,540 KgJhr. requires a gas venting rate ofabout 33.3 KgJhr. for optimum efficiency in terms of the production rateof toluene diamine product and losses of unreactive hydrogen gas in thevented gases. Venting of gases is, of course, essential to prevent unduebuild-up of impurities in the reactor with consequent adverse effectupon the production rate of desired product. Gas vented under the abovesteady state conditions will contain about 10-15 mole percentimpurities.

The computer utilized can be any general-purpose analog computer whichcan be programmed to solve equation (1) above. An Electronics AssociatesModel No. TR48 analog computer has been found to be satisfactory butother similar apparatus can be used.

The operation of this invention can be understood from the followingdiscussion which clarifies the application of feedforward and feedbackconcepts of control. ln feed-forward control, the hydrogen flow rate isproportioned stochiometrically to the DNT feed rate,

as follows The operation of this feedback correction can be understoodmost clearly as follows. The vent gas rate is given by the followingmaterial balance:

X2 2 2 rm) Equation (3) can be approximated by the following equation:

v 0.132 F (X, 2)

where V, F, X,, and X are defined following equation (l) and the term Xhas the following value:

(Xi) (X, K(e (de/dl) y)] The value of V calculated from equation 3) isnot the same as that calculated from equation (4) for the condition K(e(de/dt) 'y) 0 as can be seen from Table 1 following:

TABLE Feed Rate Impurity Vent Rate, V, Excess F, Kg. Conc'n, mole Kg/hr.Hydrogen DNT/hr. X, Eq. (3) Eq. (4) 4540 0.5 13.0 33.3 23.0 1.99 45400.75 13.0 51.1 34.5 4.64 4540 0.75 25.4 33.2 17.7 1.93 5440 0.75 13.039.8 41.5 4.64

The vent rate calculated from equation (4) differs appreciably from theexact value calculated from equation (3). However, in practice, thefeedback term K(e (deldt) 'y) does not have the value zero but ratherhas a finite value such that the vent rate V calculated from thesimplified equation 4) is automatically made equal to the exact value ofV calculated from equation (3) by the control system of this invention.The net result of feedback control is that a new value of theconcentration of impurities in the hydrogen feed, X is created in orderto control the concentration of impurities in the vent gas, X to anyspecified value. Because of the adverse effect of impurities on thereaction rate, however, X in practice is controlled to 15 mole percentor less, generally in the range of 10 to 15 mole percent.

According to line 1, Table I, the normal operation is when X, and X, are0.5 and 13.0 mole percent, respectively. For this condition, the ventrate is 33.2 Kg./hr. for a DNT feed rate of 4,540 Kg./hr. lf (line 2,Table l), X, were to increase suddenly to 0.75 mole percent, the ventrate would increase by feedback control to a steady-state value of 51.1KgJhr. to maintain an impurityconcentration of 13.0 mole percent. ifthen (line 3, Table I), it were attempted to maintain the vent rateconstant at the normal value of 33.2 Kg./hr., the impurity concentrationin the reactor would increase to 25.4 mole percent, which exceeds thevalue of 15 mole percent at which the catalyst activity starts todiminish. On the other hand, if the DNT feed rate were to suddenlyincrease to 5,440 KgJhr. (see line 4, Table l), the vent flow rate wouldincrease to a steady-state value of 41 .5 Kgjhr. by feedforward controlin order to maintain steady-state operation. Under steady-stateconditions, the impurity concentration in the vent gas will contain aspecified constant value within the range to mole percent.

It should be noted in Table I that the percent excess hydrogen rangesfrom only about 2 to 5 percent in order to purge impurities. Thesignificance of this small excess can best be appreciated from the factthat the flow of vent gas is just controlled by the smallestcommercially available non-special control valve, onequarter inchdiameter iron-pipe-size, despite the fact that the DNT flow rate is solarge in magnitude, 4,540 Kg./hr. I

Finally, in equations (1), (3), and (4), the vent rate V is shown by itssquare, V. This is commonly known in control theory as flow-square" andis done because of the following facts: (1) the square of the flow rateof a fluid flowing in turbulent flow is directly proportional to thepressure drop across either a control valve or an orifice plate and (2)the pressure drop through either the control valve or orifice plate istransmitted without change to the flow controller where it is added orsubtracted from other flow rate pressure drop signals or from a setpoint. In other words, pneumatic controllers operate on pressure signalswhich in turn are proportional to the square of the flow rate. Hence,flowsquare results are directly useful in pneumatic control systemswithout need for change.

The present invention can logically be termed a feedforward-feedbacksystem of control since (a) at least one feed stream, hydrogen, isanalyzed for deviations and (b) the results are sent forward via thecomputer to the vent flow rate controller to anticipate and offsetchanges in the process conditions which otherwise would necessarilyresult. Also, the changes in vent gas composition are monitored, signalsrepresentative thereof are sent backward, advising the computer of theeffect of any upsets and changes made to conteract them. This dualcontrol system when utilized in a con-' tinuous process affordscontinuous close control of the operating conditions involving aliquid-gas reaction.

In a commercial process for hydrogenating dinitrotoluene, as well asother materials, it is customary to use as a source of hydrogen theby-product hydrogen produced in another process. The purity of thehydrogen, particularly when it is used directly with little or nointermediate purification, is dependent upon the character and operationof that otherprocess. Upsets in that process, which may be caused byequipment breakdowns or changes in reaction conditions or changes in rawmaterials can cause rather substantial changes in the purity of thehydrogen by-product used in the present invention. When the hydrogen ispurchased from another party in an over the fence arrangement(continuously produced and transmitted by pipe line from the producersplant to the users adjoining plant), upsets in purity concentration arebeyond the control of the user and he must be prepared to accommodatethese upsets or cease operations temporarily. The latter is anunacceptable alternative in case of a continuous process.

The equipment used in the control system for hydrogenatingdinitrotoluene is commercially available and it can be pneumatically orelectrically operated. The means for measuring and transmitting a signalrepresenting the feed rate of dinitrotoluene can be accomplished with aflow meter such as the Target Meter, Model 11, 18-54, manufactured byFoxboro. The means for analyzing the concentration of impurities in thegas feed and transmitting a signal representative thereof can beaccomplished by sampling with a Mason-Neilan Type No. 138-1 1 diaphragmvalve and analyzing by a Mine Safety Appliance Company Thermatron gasanalyzer. The means for measuring and transmitting the vent gas flowrate can be an orifice plate used in conjunction with a differentialpressure transmitter such as Model l3-A manufactured by Foxboro. Themeans for controlling the vent gas flow rate can be a Model 5322-TSDcontroller manufactured by Foxboro used with control valve such as No.138-1 1 manufactured by Mason-Neilan.

The following example illustrates the invention.

EXAMPLE A cylindrical stainless steel reactor is used for hydrogenatingdinitrotoluene. Both hydrogen gas at a rate of 321 Kglhr. containingabout 0.5 mole percent inert impurities (mostly methane) anddinitrotoluene at a rate of 4,540 KgJhr. are fed into the lower regionof the reactor. Liquid metatolylene diamine is drawn from the bottom ofthe reactor, most of which is recycled through a cooler before beingreturned to the top of the reactor, and the remainder removed asproduct. The tolylene diamine product flow rate is 3,040 KgJhr. Theamount of water formed in the reaction is 1,792 Kg./hr., part of whichis removed with the vent gas and the remainder with the tolylene diamineproduct. The vent gas flow depends on the concentration of impuritiestherein which preferably is about 13.0 mole percent on the basis ofwater-saturated vent gas at steady state and normal operatingconditions. Under these conditions 11.7 Kg. water perhour is removedwith the vent gas and the remainder (1,780 Kg.) with the diamineproduct.

In operation the actual flow rate of dinitrotoluene feed is sensed by anorifice place (4,540 KgJhr. normally) and a signal representing the flowrate is transmitted by a flow transmitter to an analog computer. Theconcentration of impurities in the hydrogen feed is also measured and asignal representing the hydrogen feed impurity concentration (normally0.5 mole percent) is transmitted to the analog computer. The vent gas(i.e., the gas phase in the reactor) is analyzed and a signalrepresenting its impurity level (normally 13.0 mole percent on awater-wet basis) is transmitted to the analog computer.

The analog computer is programmed to accept these signals and todetermine the desired amount of gas V) to be vented to return theprocess to the normally steady state reaction conditions or maintain itthere as the case may be. The computer utilizes the following equation:

where:

F dinitrotoluene feed rate (e.g., 4,540 KgJhr.)

X l actual impurity concentration in the feed hydrogen (0.5 mole percentnormally) X actual impurity concentration of the vent gas (13.0 molepercent at normal steady state) e error difference between the desired(normal) and the actual impurity concentration of the vent gas (ed) forsteady state conditions) mole percent K controller gain (percent pervolt) 'y process time constant of gas contained in the reactor and thevent gas system (minutes) t time (minutes) V vent gas flow rate onwater-wet basis (33.3

Kg./hr. at normal steady state).

ln this equation, controller gain K is chosen to have the value but itmay range from 1 to inclusive without affecting the steady-state valueof the vent gas flow rate. The process time constant 7 is more complex.It is, for practical purposes, the total volume of gas contained in thereactor and the vent gas system all divided by the vent gas flow rate.More exactly, the process time constant of both the gas system and theinstruments used to control the vent gas flow rate is obtained from aBode plot which is described in Techniques of Process Control by P. S.Buckley, N.Y., Wiley, 1964. In this technique, a sinusoidal variation iscaused in the feed hydrogen flow rate. The frequency is measured forboth the feed-hydrogen stream variation and the vent gas flow ratevariation and the log of this ratio is plotted against the log of thefrequency of variation in the feedhydrogen stream. The process timeconstant 7 is the reciprocal of the feed-hydrogen stream variationfrequency when the graphical relationship just mentioned is extrapolatedback to a value of zero on the ordinate scale. For illustrativepurposes, a value of 3 minutes is chosen for -y but it may range from 2to 5 minutes without affecting the steady-state vent gas flow rate.

In order to illustrate the operation of the present invention, supposethat the concentration of impurity in the hydrogen feed, X were toincrease abruptly from the normal value of 0.5 mole percent up to a newvalue of 0.75 mole percent. If the vent gas flow rate were notincreased, the reactor impurity concentration would increase to 25.4mole percent (as shown in line. 3, Table I) at which concentration thecatalyst activity would be reduced. Assume, then that the reactorimpurity concentration, X would increase from the normal value of 13.0mole percent up to a new value of 14.0 mole percent and that this changeoccurred in minutes. The error, e, is (14.0 13.0) or 1.0 percent and therate of change of the error, de/dt, is 1.0/ 15 or 0.0667 mole percentper minute. The calculations carried out by the computer to obtain thenew value of the vent rate are as follows:

The square of the vent gas flow rate is given by the equation:

which for the feed rate F of 4,540 KgJhr. dinitrotoluene is equal to8.95 X 10. From this result, Vis calculated to be 299 Kg./hr. While thevent gas flow rate would have such a large value initially, it wouldrapidly be reduced to 51.1 Kg./hr. if X 1 of the feed were to continueat the value of 0.75 mole percent or to 33.3 KgJhr. if X were to revertto its normal value of 0.5 mole percent. The factor of importance to benoted here is that the impurity concentration X, of the reactor woulddeviate little from the desired value of 13.0 mole percent because ofthe corrective action of the control system of this invention.

In operation, following an impurity upset of 0.25 mole percent in thegas feed, a signal representing the required vent gas flow ratecalculated by the computer to offset the upset (e.g., a rate of 299Kg./hr.) is automatically sent to a vent gas flow rate controller whereit is used as a set point and compared with a signal from an orificeplate which continuously measures the actual vent gas flow rate. Thecontroller then emits a signal representing the deviation of the actualflow rate from the set point and this signal is sent to the vent controlvalve to change the actual flow rate to the required flow rate. As theerror difference between the actual and the desired impurityconcentration in the reactor (vent gas) decreases, the required vent gasflow rate, as calculated in accordance with equation (1) by the analogcomputer decreases, thus causing a change in the set point of the flowrate controller in the direction of the normal flow rate. The controlsystem of this invention has been found to be capable, despite gas feedimpurity upsets involving changes of up to 40 percent in impurityconcentration, of maintaining the impurity content in the reactor gasphase within one percent of the desired value whereas prior art feedbackcontrol systems permitted variances several fold this amount.

l claim:

1. In a continuous process for reacting a liquid with an impure gas in areaction zone containing a liquid phase and a gas phase whereby liquidand gas reactants are continuously fed to the reaction zone and reactionproduct and unreacted gas are continuously removed, the improvementconsisting essentially of periodically transmitting to a computersignals representing a. the liquid feed rate b. the concentration ofimpurities in the gaseous reactant c. the concentration of impurities inthe gas phase in the reaction zone said computer being programmed togenerate a signal representative of gas removal rate V in accordancewith the equation F gas feed rate, KgJhr.

X actual impurity concentration in the feed gas,

mole percent X actual impurity concentration of the vent gas,

mole percent e error difference between the desired (normal) and theactual impurity concentration of the vent gas (e =0 for steady stateconditions), mole percent K controller gain (percent per volt) 'yprocess time constant of gas contained in the reactor and the vent gassystem (minutes) I time (minutes) V= vent gas flow rate on water-wetbasis, KgJhr. and adjusting the gas removal rate in accordance with thesignal so generated.

2. The process of claim 1 in which the computer generated signal istransmitted to a vent gas controller which automatically adjusts the gasremoval rate in accordance with the generated signal.

3. The process of claim 1 in which the reactant liquid is dinitrotolueneand the reactant gas is hydrogen.

4. The process of claim 3 in which the reactant gas fed to thereactionzone is hydrogen having a purity of at least about 98 mole percent.

5. The process of claim 4 in which the gas phase in the reaction zonehas an impurity content of about 10-15 percent.

6. Apparatus for controlling the efficiency of a reaction between aliquid and a gas phase comprising a. a closed reactor fitted with inletports for reactant gas and reactant liquid and exit ports for reactionproduct and unreacted reactant gas,

tor,

f. computer means adapted for collecting the above signals andprogrammed to compute therefrom a flow rate for gas leaving the reactorto maintain the impurity content of the gas phase in the reaction zonewithin predetermined limits, and

g. means for adjusting the flow rate of gas leaving the reactor to theflow rate prescribed by the computer.

1. In a continuous process for reacting a liquid with an impure gas in areaction zone containing a liquid phase and a gas phase whereby liquidand gas reactants are continuously fed to the reaction zone and reactionproduct and unreacted gas are continuously removed, the improvementconsisting essentially of periodically transmitting to a computersignals representing a. the liquid feed rate b. the concentration ofimpurities in the gaseous reactant c. the concentration of impurities inthe gas phase in the reaction zone said computer being programmed togenerate a signal representative of gas removal rate V in accordancewith the equation V2 (0.132)2F2 (X1 + K(e + (de/dt) gamma ))2/(X2)2where: F gas feed rate, Kg./hr. X1 actual impurity concentration in thefeed gas, mole percent X2 actual impurity concentration of the vent gas,mole percent e error difference between the desired (normal) and theactual impurity concentration of the vent gas (e 0 for steady stateconditions), mole percent K controller gain (percent per volt) gammaprocess time constant of gas contained in the reactor and the vent gassystem (minutes) t time (minutes) V vent gas flow rate on water-wetbasis, Kg./hr. and adjusting the gas removal rate in accordance with thesignal so generated.
 2. The process of claim 1 in which the computergenerated signal is transmitted to a vent gas controller whichautomatically adjusts the gas removal rate in accordance with thegenerated signal.
 3. The process of claim 1 in which the reactant liquidis dinitrotoluene and the reactant gas is hydrogen.
 4. The process ofclaim 3 in which the reactant gas fed to the reaction zone is hydrogenhaving a purity of at least about 98 mole percent.
 5. The process ofclaim 4 in which the gas phase in the reaction zone has an impuritycontent of about 10-15 percent.
 6. Apparatus for controlling theefficiency of a reaction between a liquid and a gas phase comprising a.a closed reactor fitted with inlet ports for reactant gas and reactantliquid and exit ports for reaction product and unreacted reactant gas,b. means for measuring and transmitting a signal representative of therate of flow of reactant liquid to the reactor, c. means for measuringand transmitting a signal representative of the concentration ofimpurities in the flow of reactant gas to the reactor, d. means formeasuring and transmitting a signal representative of the concentrationof impurities in the gas phase of the reactor, e. means for measuringand transmitting a signal representative of the flow rate of gas leavingthe reactor, f. computer means adapted for collecting the above signalsand programmed to compute therefrom a flow rate for gas leaving thereactor to maintain the impurity content of the gas phase in thereaction zone within predetermined limits, and g. means for adjustingthe flow rate of gas leaving the reactor to the flow rate prescribed bythe computer.